Pipes of this kind are mainly used where hot gases have to be passed from a heating point to a working point and then returned in a closed circuit, as for example in a gas turbine, to the heating point. The hot gas is carried in the circular inner pipe and the cold gas, for example helium, in the circular annular outer pipe. A pipe construction of this kind has above all the advantage that it considerably reduces the loss of heat; in addition, only one supporting pipe structure is necessary.
For devices for closing coaxial pipes of this kind, only one proposal is known, and this is scarcely practicable since it does not provide a safety zone in the coaxial flow system which, in the event of failure of the device, can separate portions of flowing gas axially away from one another and thus prevent undesirable gas flow. Thus the known proposal does not provide a means by which the flow of hot gas can be rapidly shut off in the event of pipe fracture.
For most closed gas circuits it would be desirable to provide such a safety zone; a closure device of this kind is however indispensable when the heating of the gas takes place in a nuclear reactor, because then it must without fail be possible to shut off the flow of hot gas in both directions of flow, so that radioactive particles cannot pass out in the event of the fracture of a pipe or heat exchanger.